I was hit with my first not too small denial of service attack. The UDP packets came from chikna.net (but this may have been forged) and were arriving at the rate of 20,000 packets per second. Now that it's over, my provider told me that it used 80GB and I probably have to pay for that bandwidth.

Is there anything I could do next? Report it somewhere or anything? I'm totally at a loss and not especially happy about having to pay $400 for an attack.

Please post any advice you may have.

Welcome to the internet. DoS attacks suck. Not much you can do. I suggest either
a) getting new server ips
b) having your ISP put you behind some sort of firewall, or have them help you out hardware wise for protection.
c) install some software-based firewalls.
or get some good iptables rulesets or ipchains rulesets

Block ALL UDP
except to your name servers

block icmp
firewall ports

and hope it doesn't happen again.